In terms of support, I don't know if the same hard-and-fast rule occurs in the US, but here they almost always have to charge whatever the headliners are charging. It's why you should never, for example, buy the shirt of a band supporting Iron Maiden, because the same shirts will probably be cheaper at their own headlining shows, or when they support someone who charges less for their own shirts.

I think that's what caused a bit of a feud between Rob Zombie and Bullet for my Valentine when they supported him in the US and Rob forced BFMV to charge $40 for tshirts, and they refused and got kicked off the tour or something.